fix: adversarial review fixes — failure shape, CHANGELOG guard, vet coherence

Claude adversarial subagent findings (11), all verified before applying:
- The dispatch JSON contract gains an explicit FAILURE shape
  ({"error":...}) and a parent branch for it — a doc-sync that could not
  run (spawned marking failed, broken preamble) previously had only the
  no-updates shape to emit, which the parent printed as 'Documentation is
  current': a silent false-clean of exactly the VAS-449 class.
- Scope guard now covers CHANGELOG: skip Step 5 voice polish and resolve
  CHANGELOG-touching gates to leave-as-is (the parent authored the
  release entry; the prompt's older auto-choose clause conflicted with
  the contract's never-rewrite invariant).
- Recovery vet is sequence-coherent: pushing a commit pushes its
  ancestors, so ANY non-doc commit (VERSION, package.json, CHANGELOG.md)
  blocks the whole sequence — no more push-the-vetted-child-of-an-
  unvetted-parent hole.
- Steps 7/8/10 failure branches stop a still-running backgrounded task
  before falling back, so a late result never races the inline audit.
- 'Documentation synced' print gated on pushed:true (item 6 owns the
  local-only outcome); doc-review skip note's backward step pointer
  fixed; foreground note scoped 'at initial dispatch' so the sanctioned
  inline FALLBACKS in Steps 7/8 read as sanctioned; constants docstring
  no longer overclaims single-sourcing (names the 3 inline templates).
- Runtime-hook TODO raised P2 → P1: the spawned trust chain is
  agent-self-asserted; prose cannot close it, the hook can.

Both docsync gate E2Es re-verified green on the amended prompt, including
the strict run_in_background === false dispatch assert.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ship/sections/test-coverage.md

@@ -4,7 +4,7 @@
 
 **Dispatch this step as a subagent** using the Agent tool with `subagent_type: "general-purpose"`. The subagent runs the coverage audit in a fresh context window — the parent only sees the conclusion, not intermediate file reads. This is context-rot defense.
 
-**Foreground required:** pass `run_in_background: false` on the Agent call — subagents run in the BACKGROUND by default since Claude Code v2.1.198. (Merely omitting the flag no longer produces a foreground run; it must be explicitly false.) Dispatch with the Agent tool itself — never substitute the Skill tool or inline execution to "guarantee" blocking: the explicit flag already makes the Agent call block, and running the work inline forfeits the fresh-context isolation this dispatch exists for. The parent needs this audit's LAST-line JSON before continuing.
+**Foreground required:** pass `run_in_background: false` on the Agent call — subagents run in the BACKGROUND by default since Claude Code v2.1.198. (Merely omitting the flag no longer produces a foreground run; it must be explicitly false.) Dispatch with the Agent tool itself — never substitute the Skill tool or inline execution at initial dispatch to "guarantee" blocking: the explicit flag already makes the Agent call block, and running the work inline forfeits the fresh-context isolation this dispatch exists for. The parent needs this audit's LAST-line JSON before continuing.
 
 **Subagent prompt:** Pass the following instructions to the subagent, with `<base>` substituted with the base branch:
 
@@ -261,6 +261,6 @@ Repo: {owner/repo}
 3. Embed `diagram` verbatim in the PR body's `## Test Coverage` section (Step 19).
 4. Print a one-line summary: `Coverage: {coverage_pct}%, {gaps} gaps. {tests_added.length} tests added.`
 
-**If the subagent fails, times out, returns invalid JSON, or never completes (backgrounded despite the flag, or no final output after ~10 minutes — stop waiting):** Fall back to running the audit inline in the parent. Do not block /ship on subagent failure — partial results are better than none.
+**If the subagent fails, times out, returns invalid JSON, or never completes (backgrounded despite the flag, or no final output after ~10 minutes — stop waiting; if a backgrounded task is still running, stop it first so a late result never races the fallback):** Fall back to running the audit inline in the parent. Do not block /ship on subagent failure — partial results are better than none.
 
 ---